Due to the fact that the instant patent application deals with semiconductor alloy materials which will be referred to by specialized definitions of amorphicity and crystallinity, it is necessary to particularly set forth those specialized definitions at the outset.
The term "amorphous", as used herein, is defined to include alloys or materials exhibiting long range disorder, although said alloys or materials may exhibit short or intermediate range order or even contain crystalline inclusions. As used herein the term "microcrystalline" is defined as a unique class of said amorphous materials characterized by a volume fraction of crystalline inclusions, said volume fraction of inclusions being greater than a threshold value at which the onset of substantial changes in certain key parameters such as electrical conductivity, band gap and absorption constant occurs. It is to be noted that pursuant to the foregoing definitions, the microcrystalline, p-doped semiconductor alloy material of the instant invention falls within the generic term "amorphous".
The concept of microcrystalline materials exhibiting a threshold volume fraction of crystalline inclusions at which substantial charges in key parameters occur, can be best understood with reference to the percolation model of disordered materials. Percolation theory, as applied to microcrystalline disordered materials, analogizes properties such as the electrical conductivity manifested by microcrystalline materials, to the percolation of a fluid through a non-homogeneous, semi-permeable medium sucn as a gravel bed.
Microcrystalline materials are formed of a random network which includes low conductivity, highly disordered regions of material surrounding randomized, highly ordered crystalline inclusions having high electrical conductivity. Once these crystalline inclusions attain a critical volume fraction of the network, (which critical volume will depend, inter alia, upon the size and/or shape and/or orientation of the inclusions), it becomes a statistical probability that said inclusions are sufficiently interconnected so as to provide a low resistance current path through the network. Therefore at this critical or threshold volume fraction, the material exhibits a sudden increase in conductivity. This analysis (as described in general terms relative to electrical conductivity herein) is well known to those skilled in solid state theory and may be similarly applied to describe additional physical properties of microcrystalline materials, such as optical gap, absorption constant, etc.
The onset of this critical threshold value for the substantial change in physical properties of microcrystalline materials will depend upon the size, shape and orientation of the particular crystalline inclusions, but is relatively constant for different types of materials. It should be noted that while many materials may be broadly classified as "microcrystalline", those materials will not exhibit the properties we have found advantageous for the practice of our invention unless they have a volume fraction of crystalline inclusions which exceeds the threshold value necessary for substantial change. Accordingly, we have defined "microcrystalline materials" to include only those materials which have reached the threshold value. Further note that the shape of the crystalline inclusions is critical to the volume fraction necessary to reach the threshold value. There exist 1-D, 2-D and 3-D models which predict the volume fraction of inclusions necessary to reach the threshold value, these models being dependent on the shape of the crystalline inclusions. For instance, in a 1-D model (which may be analogized to the flow of charge carriers through a thin wire), the volume fraction of inclusions in the amorphous network must be 100% to reach the threshold value. In the 2-D model (which may be viewed as substantially conically shaped inclusions extending through the thickness of the amorphous network), the volume fraction of inclusions in the amorphous network must be about 45% to reach the threshold value. And finally in the 3-D model (which may be viewed as substantially spherically shaped inclusions in a sea of amorphous material), the volume fraction of inclusions need only be about 16-19% to reach the threshold value. Therefore, amorphous materials (even materials classified as microcrystalline by others in the field) may include crystalline inclusions without being microcrystalline as that term is defined herein.
Amorphous thin film semiconductor alloys have gained growing acceptance as a material from which to fabricate electronic devices such as photovoltaic cells, photoresponsive and photoconductive devices, transistors, diodes, integrated circuits, memory arrays and the like. This is because the amorphous thin film semiconductor alloys (1) can be manufactured at relatively low cost, (2) possess a wide range of controllable electrical, optical and structural properties and (3) can be deposited to cover relatively large areas.
Recently, considerable effort has been expended to develop systems and processes for depositing amorphous semiconductor alloy materials which encompass relatively large areas and which can be doped so as to form p-type and n-type semiconductor alloy layers for the production therefrom of thin film electronic devices, particularly thin film p-n type and p-i-n type photovoltaic devices which would be substantially operatively equivalent or superior to their crystalline counterparts. Among the investigated semiconductor alloy materials of the greatest significance are the silicon, germanium, and silicon-germanium based alloys. Such semiconductor alloys have been the subject of a continuing development effort on the part of the assignee of the instant invention, said alloys being utilized and investigated as possible candidates from which to fabricate amorphous semiconductor, electronic and photresponsive devices.
As disclosed in U.S. Pat. No. 4,226,898 of Ovshinsky, et al, which patent is assigned to the assignee of the instant invention and the disclosure of which is incorporated herein by reference, fluorine introduced into the silicon alloy semiconductor layers operates to substantially reduce the density of the localized defect states in the energy gap thereof and facilitates the addition of other alloying materials, such as germanium. As a result of introducing fluorine into the host matrix of the semiconductor alloy, the film so produced can have a number of favorable attributes similar to those of single crystalline materials. A fluorinated thin film semiconductor alloy can thereby provide high photoconductivity, an increased number of charge carriers, low dark intrinsic electrical conductivity, and, where desired, such alloys can be modified to help shift the Fermi level to provide substantially n- or p-type extrinsic electrical conductivity. Thus, fluorinated thin film amorphous semiconductor alloy materials can be fabricated in a manner which allows them to act like crystalline materials and be useful in devices, such as, solar cells and current controlling devices including diodes, transistors and the like.
Unlike crystalline silicon which is limited to batch processing for the manufacture of solar cells, the aforedescribed amorphous silicon and germanium alloys can be deposited in multiple layers over large area substrates to form semiconductor devices in a high volume, continuous processing system, Such continuous processing systems are disclosed in the following U.S. Pat. Nos. 4,400,409, for A Method Of Making P-Doped Silicon Films and Devices Made Therefrom and No. 4,410,588, for Continuous Amorphous Solar Cell Production System; No. 4,438,723, for Multiple Chamber Deposition And Isolation System And Method. As disclosed in these patents, a substrate may be continuously advanced through a succession of interconnected, environmentally protected deposition chambers, wherein each chamber is dedicated to the deposition of a specific semiconductor material. In making a photovoltaic device, for instance, of p-i-n type configurations, the first chamber is dedicated for depositing a p-type semiconductor alloy, the second chamber is dedicated for depositing an intrinsic amorphous semiconductor alloy, and the third chamber is dedicated for depositing an n-type semiconductor alloy. The layers of semiconductor alloy material thus deposited in the vacuum envelope of the depostion apparatus may be utilized to form photoresponsive devices, such as, but not limited to photovoltaic cells which include one or more p-i-n type cells. Note that as used herein the term "p-i-n type" will refer to any sequence of p and n or p, i, and n semiconductor alloy layers. Additionally, by making multiple passes through the succession of deposition chambers, or by providing an additional array of deposition chambers, multiple stacked cells of various configurations may be obtained.
The concept of utilizing multiple stacked cells, to enhance photovoltaic device efficiency, was described at least as early as 1955 by E. D. Jackson in U.S. Pat. No. 2,949,498 issued Aug. 16, 1960. The multiple cell structures therein disclosed were limited to the utilization of p-n junctions formed by single crystalline semiconductor devices. Essentially, the concept employed different band gap devices to more efficiently collect various portions of the solar spectrum and to increase open circuit voltage (Voc). The tandem cell device (by definition) has two or more cells with the light directed serially through each cell. In the first cell, a large band gap material absorbs only the short wavelength light, while in subsequent cells, smaller band gap materials absorb the longer wavelengths of light which pass through the first cell. By substantially matching the generated currents from each cell, the overall open circuit voltage is the sum of the open circuit voltage of each cell, while the short circuit current thereof remains substantially constant. Such tandem cell structures can be economically fabricated in large areas by employing thin film amorphous, microcrystalline, and polycrystalline semiconductor alloy materials, such as the microcrystalline p-doped semiconductor alloy material of the instant invention.
It is now possible to manufacture high quality n-doped and intrinsic thin film semiconductor alloy layers utilizing techniques developed by the assignee of the instant invention. However, the p-doped thin film semiconductor alloy layers heretofore fabricated have, in many instances, been of less than the optimum quality required for the manufacture of the highest efficiency semiconductor alloy devices therefrom. Accordingly, because of the limitations imposed by the p-doped semiconductor alloy material, the optimum operational potential of many classes of thin film semiconductor alloy devices have as yet to be achieved.
We have recently discovered that if a highly transparent, wide band gap, microcrystalline, p-doped semiconductor alloy layer (also referred to as a highly "p-doped layer") could be fabricated, p-i-n and n-i-p type photovoltaic cells and particularly p-i-n and n-i-p tandem photovoltaic cells manufactured with said microcrystalline, p-doped semiconductor alloy layer will exhibit not only significant, but synergistic improvement in the operational performance thereof. Such a highly p-doped microcrystalline semiconductor alloy layer would have a low activation energy and would thus increase the magnitude of the electrical field established across the intrinsic semiconductor alloy layer by itself and the oppositely disposed n-doped layer, thereby improving the fill factor of the photovoltaic cell fabricated therefrom. Similarly, the built-in potential of the photovoltaic cells, and consequently, the open circuit voltage generated thereacross would be increased by the presence of the highly p-doped, microcrystalline, semiconductor alloy layer. Also, since the built-in potential is increased, charge carriers are more readily moved from the photoactive region in which they are generated to the respective electrodes despite the presence of photoinduced defects which are responsible for the well known effect of Staebler/Wronski degradation, thereby providing drastically improved stability. The improved electrical conductivity of microcrystalline p-doped semiconductor alloy material, vis-a-vis similarly constituted and doped semiconductor alloy material, which material is characterized by a number of crystalline inclusions below the aforementioned threshold value, would further provide for decreased series resistance encountered by charge carriers in their movement through the photovoltaic cell. The decrease in series resistance would result in improved fill factor and overall efficiency of that photovoltaic cell.
Wide band gap, p-doped microcrystalline semiconductor alloy layers are more optically transparent than corresponding amorphous semiconductor alloy layers which have a volume fraction of inclusions below the threshold value. We came to the analytical and theoretical conclusion that such transparency is desirable, if not essential, in the p-doped layer of a p-i-n type photovoltaic cell because the increased transparency will allow more light, whether incident upon the p-doped layer or redirected by a back reflector through that p-doped layer, to pass therethrough for absorption in the intrinsic semiconductor alloy layer (the photoactive region) of the photovoltaic cell. It is in this intrinsic semiconductor alloy layer that charge carrier pairs are most efficiently generated and separated. Therefore, photovoltaic cells employing microcrystalline, wide band gap, p-doped layers of semiconductor alloy material would also produce higher short circuit currents. This consideration of transparency would be especially signficant for a tandem p-i-n type photovoltaic device, described hereinabove, which device is formed of a multiplicity of stacked, individual p-i-n type photovoltaic cells. This is because, we theorized, in such a tandem photovoltaic device, a light absorbing (narrow band gap) p-doped layer in (1) the upper photovoltaic cell would "shade" one or more of the underlying cells and thus reduce the amount of incident light absorbed in the intrinsic semiconductor alloy layer, the layer with the longest lifetime for charge carriers thereof, and (2) the lower photovoltaic cell would "shade" one or more of the superposed cells and thus reduce the amount of redirected light absorbed in the intrinsic semiconductor alloy layer.
We went on to hypothecate, if it would be possible to fabricate a microcrystalline p-doped semiconductor alloy material having a wide band gap, high electrical conductivity and low activation energy, said p-doped semiconductor alloy material would prove to be very beneficial in the manufacture of photovoltaic devices, especially tandem photovoltaic devices. Similarly, such a p-doped microcrystalline semiconductor alloy material could be advantageously employed in the manufacture of other electronic devices to complement the presently available high conductivity n-type thin film silicon alloy semiconductor material. Obviously, high quality, microcrystalline, p-doped semiconductor alloy materials would have immediate utility in the fabrication of a wide variety of thin film electronic devices such as thin film transistors, diodes, memory arrays and the like. Simply stated, such p-doped microcrystalline semiconductor alloy material could be made to exhibit the high conductivity and wide band gap characteristics of corresponding single crystal semiconductor material and could be made to accept sufficiently high levels of p-dopant material to provide a low activation energy. Further, such p-doped microcrystalline semiconductor alloy materials could be produced in a wide range of compositional variations by low cost vapor deposition techniques.
We are aware that microcrystalline alloy materials have been known for some time, and various reseachers have reported a wide variety of microcrystalline semiconductor alloy materials and methods for their fabrication. We are not implying that we have invented the concept of microcrystalline semiconductor material per se. However, we are claiming to have recognized the previously unrecognized facts that (1) the particularly advantageous properties exhibited by microcrystalline semiconductor alloy materials can be further enhanced by the inclusion of the "super-halogen" fluorine into the silicon:hydrogen semiconductor alloy matrix; and (2) microcrystalline semiconductor alloy materials (as we have described them hereinabove), with or without the addition of fluorine, have particular utility in the fabrication of n-i-p type photovoltaic devices, particularly tandem n-i-p photovoltaic devices, wherein said materials may be synergistically incorporated into the device structure to provide a photovoltaic device having uniquely high efficiency and stability. The importance of these two discoveries will be individually treated in the succeeding sections.